Studying people who have brain damage or illness has been hugely important to progress in psychology. The approach is akin to reverse engineering: study how things go wrong when particular regions of the brain are compromised and it provides useful clues as to how those regions usually contribute to healthy mental function.

As a result, some neuropsychological conditions, such as Broca’s aphasia (speech deficits), prosopagnosia (a difficulty recognising faces, also known somewhat misleadingly as “face blindness”) and Alien Hand syndrome (a limb seeming to act of its own volition) have become extremely well-known – at least in psychological circles – and extensively studied. However, others are virtually unheard of, even though their importance to our understanding of the brain is significant.

Neuropsychologist Alfredo Ardila at Florida International University has just published in the journal Psychology and Neuroscience an overview of four of these little-known conditions, “so rare that they are not even mentioned in basic neuropsychology textbooks”: Central achromatopsia, Bálint’s syndrome, Pure-word deafness, and aphasia of the supplementary area. This follows a paper he published last year covering four other rare but important neuropsychological syndromes: Somatoparaphrenia, Akinetopsia, Reduplicative Paramnesia, Autotopagnosia.

“In neuropsychology … there are some unusual syndromes that are found very sporadically,” he writes. “But their rarity does not diminish their importance in the fundamental understanding about the brain organisation of cognition, as well as in clinical analysis of patients with brain pathologies.”

Here’s a brief breakdown of what Ardila has to say about these rare conditions and why they’re important.

Akinetopsia
Imagine if someone pressed pause on your view of the world: that may be what’s it’s like for patients with akinetopsia, who are unable to see movement, even though they can perceive the sound of motion. The syndrome is associated with damage to a specific region of the brain’s visual cortex (area V5) and it can be induced in healthy volunteers by using transcranial magnetic stimulation to induce a “virtual lesion” in that same brain region. Akinetopsia is extremely rare but may be associated with the Zeitraffer phenomenon, which is when patients perceive the speed of moving objects as noticeably slowed down. “The description of akinetopsia represents a crucial step in the comprehension of how motion perception is organized in the brain,” notes Ardila.

Aphasia of the Supplementary Motor Area
In this condition, patients have a particular problem speaking spontaneously. They can repeat things back when asked, and name things when prompted, but they don’t seem to be able to initiate speech. The region of the brain that’s affected – the Supplementary Motor Area (SMA) – is involved in preparing and initiating movements, especially sequences of movements, and it’s thought this form of aphasia (arising from damage to the SMA) is due to problems with planning the movements required for spontaneous speech, rather than a language problem per se. Language comprehension is mostly unaffected. “Aphasia of the SMA illustrates the complex overlap between language and motor processes, which has contributed positively to further our understanding about brain representation of linguistic processes,” Ardila said.

Autotopagnosia
This is the inability to locate and identify parts of one’s body even though the ability to locate and name the parts of objects, plants or animals is unaffected. The condition can be seen as an impairment in updating the body schema – the brain’s representation of the body and where it is located in space – combined with a naming impairment, suggesting that language deficits are involved. The condition is usually associated with damage to the rear of the parietal cortex (near the crown of the head) in the left hemisphere. “Although autotopagnosia was described over a century ago, research has been limited,” says Ardila.

Bálint’s syndrome
Also first described over a century ago, people with this condition often behave as if they are blind. Associated with damage or pathology to the parietal and occipital lobes, this syndrome is really a complex involving three sub-syndromes: simultanagnosia which is the inability to perceive an entire scene, even while being able to see individual elements within it; optic ataxia, which is a problem reaching to specific objects even though the patient has no muscular or motor problems per se; and gaze apraxia, which describes the way the patient will struggle to move their eyes voluntarily or to keep fixated on an object. Ardila says “this is one of the clinical syndromes which has most contributed to the understanding of visual and spatial perception.”

Central Achromatopsia
This is the loss of the ability to perceive colours caused entirely by damage to parts of the brain, usually in the occipital or temporal lobe (located near the back and sides of the head), rather than to any problem with the eyes. It can co-occur with other conditions such as prosopagnosia, but importantly the ability to imagine or visualise colours is preserved. “Evidently this syndrome is crucial to understand the brain organisation of visual perception, particularly colour perception,” writes Ardila.

Pure-word Deafness
Patients with this condition display a profound problem understanding spoken language, even though their ability to read and to speak is completely unaffected. Patients describe speech sounding as if it is a barely audible whisper or as if it is a foreign language. Unlike other conditions that affect language comprehension, such as Wernicke’s aphasia, pure-word deafness is considered a purely perceptual impairment and consistent with this, it is often associated with lesions to auditory processing regions in the brain.

Reduplicative Paramnesias
This extraordinary condition leads patients to believe that a particular place, such as their home or even an entire city, has been duplicated and is located in two places at once. Less often, patients may express the same belief about a person, a particular object or a body part. Forms of reduplicative paramnesia have some similarities with other delusional syndromes to do with identity, such as capgras syndrome in which the patient believes one or more of their relatives or friends have been replaced with imposters; Frégoli syndrome, the belief that someone, usually a perceived stalker, is repeatedly changing their appearance; and intermetamorphosis – the belief that someone has transformed into another person. The evidence suggests that reduplicative paramnesias for place are often associated with damage to the right hemisphere of the brain, and are perhaps more likely due to this organic damage, whereas Ardila proposes that the other delusional identity syndromes may more often have a psychological cause.

Somatoparaphrenia
First described in 1942, patients with this syndrome deny owning one or more of their limbs on the opposite side of their body to where they have damage in their brains. The syndrome tends to co-occur with another syndrome known as anosognosia, which is a denial of an impairment, such as paralysis to a limb. Somatoparaphrenia can also be seen as a more extreme version of hemiasomatognosia, which is a lack of awareness of one’s own limb(s). Curiously, patients with somatoparaphrenia may recognise an affected limb as theirs when they see it in a mirror. Ardila says the condition, which has been associated with lesions to multiple regions in the brain, is little understood and may partly be a psychiatric condition (a form of delusion) as well as neurological. “In this regard, the analysis of somatoparaphrenia may contribute to further our understanding about the neurological bases of psychosis,” he writes.

It seems, that every single specialized faculty of our intrinsic and extrinsic awareness, is made up of a plethora neurologically specific, interactive, specializing tissues, either acting in consort, or sequestered but linked, for our existential self.